Polarization measurements such as Polarization Mode Dispersion (PMD)/Differential Group Delay (DGD) and Polarization Dependent Loss (PDL) play a critical role in the design of current high-speed optical communication systems. These parameters are usually determined by algorithms that require a set of measurements for a number of polarization states. In general, it is desired to measure both DGD and PDL as a function of wavelength since the parameters typically vary with wavelength for an optical device.
In conventional measurement systems, a separate scan for each polarization state across the wavelength region of interest is required. Furthermore, averaging of multiple scans for each state may be necessary to achieve the desired signal-to-noise ratio in the measurement. This can yield relatively long measurement times during which the device must not be disturbed. In addition, long measurement times can limit the manufacturing throughput.
There are known several methods to determine the PDL and PMD/DGD of an optical device. The most important techniques are based on applying a set of well-known polarization states to the device under test (DUT) and measuring the transfer function (amplitude and phase of the signal as a function of optical wavelength) at the receiver for each of these states. In the so-called Mueller method (C. Hentschel and D. Derrickson in Fiber Optic Test and Measurement, ed: D. Derrickson, Prentice Hall, pp.356+, 1998), the PDL of the DUT can be calculated from the measured optical power transmission at linear horizontal (0°), linear diagonal (45°), linear vertical (90°) and right-hand circular polarization states only. Similarly, in the method developed by Williams (P. A. Williams, Electronic Letters, 36, 1578 (1999), the DGD of the DUT is derived from phase delay measurements at these four states only. Generally, a separate measurement is performed for each of the four polarization states.
Related solutions are disclosed in European publications EP 1191320A1 and EP 1207377A2.
A method allowing the determination of polarization dependent parameters in an interferometric approach by measuring only two orthogonal polarization states in two detectors simultaneously, has recently been proposed in European patent EP 1113250 and in a paper by Van Wiggeren (G. D. Van Wiggeren et al., OFC 2002 Technical Digest pp. 253). The invoked algorithm is based on the assumption that the DGD of the DUT is, to the first approximation, independent of wavelength.
One of the objects of the invention is to provide an apparatus and algorithm that allow the determination of both parameters, DGD and PDL, in a single wavelength sweep using a single detector without this approximation.